1. Field of the Invention
The present invention relates to a method for manufacturing of multilayer ceramic substrates used for circuit boards in electronic computers, electronic devices such as electronic exchangers and the like In particular, the present invention relates to a method for manufacturing of multilayer ceramic substrates and for manufacturing of multilayer glass.ceramic substrates by sintering green sheet laminates prepared using organic binders and conductor materials of low melting point such as copper.
2. Description of Related Art
Hitherto, multilayer ceramic substrates have been produced using mainly alumina as an insulating material and molybdenum or tungsten as a conductor material which is a high-melting point metal capable of being sintered together with alumina. However, these conductor materials suffer from the problems that they are high in electric resistance and furthermore, a high firing temperature of about 1600.degree. C. is required for manufacture of the ceramic substrates, consuming a large quantity of energy.
Under the circumstances, there has been proposed a method for manufacturing multilayer ceramic substrates which uses copper having a relatively low melting point and a low electric resistance as a conductor material and glass.ceramics having a firing temperature of lower than 1000.degree. C. as a material for ceramic substrates (JP 1-50120B). According to this method, green sheets are fired for a long time in an atmosphere where the binder contained in the green sheets is oxidized, but copper is not oxidized, for example, an atmosphere where concentrations of steam, nitrogen gas and hydrogen gas are severely controlled. Moreover, depolymerization type resins are used as the organic binders used for preparing green sheets so that the binders can be easily removed by the firing in the above-mentioned atmosphere.
As related techniques, mention may be made of JP 1-50120B and JP 2-49550B.
In the manufacture of multilayer ceramic substrates, via-holes for electric connection between the upper and lower layers are made through green sheets and in addition, conductor circuit patterns are printed on the green sheets. Therefore, good punchability of via-holes; high positional accuracy in punching of via-holes and small deformation of green sheet after printing of the conductor are all necessary characteristics in manufacture of multilayer ceramic substrates. Furthermore, from the point of hygienic safety of the environment, the binders used with water as a solvent are preferable to the binders used with organic solvents. Thus, such firing processes being able to freely select the binders irrespective of the atmosphere is advantageous as compared with the above-mentioned conventional firing method carried out in the specific atmospheres.
Further, it is advantageous for improvement of productivity that the firing atmosphere does not need severe control and the binder can be removed in a short time. However, the conventional firing for removal of binder requires, for example, about 100 hours at about 800.degree. C. in a non-oxidizing atmosphere, namely, a firing time of 4-5 days for removal of binder in order to reduce the residual carbon content to 200 ppm or lower. Thus, improvement in this respect has been demanded.